Introduction Catalyst loss of activity with time-, i.e. “deactivation”. Catalyst have only limited lifetime Also known as Ageing catalyst activity is defined as

Catalyst deactivation is the result of number of unwanted chemical and physical changes

Decline in activity is due to Blocking of the catalytically active sites Loss of catalytically active sites due to chemical, thermal or

mechanical processes

Types of Catalyst Deactivation

Catalysts frequently lose an important fraction of their activity while in operation.Three causes for deactivation:

a. Structural changes in the catalyst itself. These changes may result from a migration of components under the influence of prolonged operation at high temperatures, for example, so that originally finely dispersed crystallites tendto grow in size.

Important temperature fluctuations may cause stresses in the catalyst particle, which may then disintegrate into powder with a possible destruction of its fine structure.

b. Essentially irreversible chemisorption of some impurity in the feed stream,which is termed poisoning.

c. Deposition of carbonaceous residues from a reactant, product or some intermediate, which is termed coking.

Time-Scale of Deactivation10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8

HydrocrackingHDS

Catalytic reforming

EO

HydrogenationsAldehydes

AcetyleneOxychlorination

MAFormaldehyde

NH3 oxidationSCR

Fat hardening

Time / seconds TWC

10 -1

10 0

10 1

10 2

10 3

10 4

10 5

10 6

10 7

10 8

1 year1 day1 hour

C3 dehydrogenation

FCC

Most bulk processes0.1-10 year

Most bulk processes0.1-10 year

Batch processeshrs-days

Batch processeshrs-days

Tailored Reactor and Process Design

Relation between time-scale of deactivation and reactor type

Time scale Typical reactor/process type

years fixed-bed reactor;

no regeneration

months fixed-bed reactor;

regeneration while reactor is off-line

weeks fixed-bed reactors in swing mode, moving-bed reactor

minutes - days fluidised-bed reactor, slurry reactor;

continuous regeneration

seconds entrained-flow reactor with continuous regeneration

Cause of Catalyst Deactivation

Four causes of Catalyst Deactivation Poisoning of the catalyst Deposits on the Catalyst Surface( Fouling, coking) Thermal Processes and sintering Catalyst loss via Gas Phase

Causes of Catalyst Deactivation

Poisoning of a Catalyst

Loss of activity due to strong chemisorptions on active sites of impurities present in the feed stream.

In heterogeneous catalysis the ‘poison’ molecules are absorbed more strongly to the catalyst surface than the reactant molecules, the catalyst becomes inactive.

Modify the nature of active sites

P P PA B A B C D C D

P

Poisons of Industrial Catalysts

Process Catalyst Poison

Ammonia Synthesis Fe CO, CO2, H2O, C2H2, S,P

Steam reforming Ni/Al2O3 H2S,As,HCl

Methanol Synthesis Cu H2S, AsH3, HCl

Catalytic Cracking SiO2-Al2O3, Zeolite NH3, Na, heavy Metals

CO hydrogenation Ni, Co, Fe H2S, COS, As, HCl

Oxidation V2O5 As

Ethylene to Ethylene Oxide Ag C2H2

Poisons Classification

Poisons can be Classified as Selective and Non Selective

Reversible or Irreversible

Example : Reversible Poisoning is due to Oxygen Compounds (O2,H2O,CO,CO2) and irreversible Poisoning is connected with non metals such as S, Cl, As Ph

EXAMPLES OF POISONING OF CATALYSTS

Leaded petrol cannot be used in cars fitted with a catalytic converter since lead strongly absorbs onto the surface of the catalyst

Cannot use copper or nickel in a catalytic converter on a car instead of the expensive platinum or Rhodium. REASON :- Any SO2 present in the exhaust fumes (trace amounts ) would poison the catalyst

Once the catalytic converter has become inactive it cannot be regenerated

Preventing Poisoning

Decrease poison Content in feed

E.g. hydrodesulphurization followed by H2S adsorption to remove sulphur Compounds

Catalyst Formulations and Design

e.g. Cu-Based Methanol Synthesis are strongly poisoned by Sulphur

KINETICS The adjustment for the decay of the catalysts:

The reactions are divided into two categories separable kinetics

non separable kinetics

)()( '' catalystfreshrhistorypastar AA

),('' catalystfreshhistorypastrr AA

a (t)

t

1.0Rate of Catalyst decay, rd

First Order Decay , p(a)=a

Second Order Decay, p(a) = a2

)]([ tapdt

dard

Poisoning Impurity P in feed Stream

Assume rate of removal of gas stream onto catalyst sites is proportional to the Number of sites that are unpoisoned and conc of poison in gas phase

BBAA

AA

CKCK

kCtar

SBSB

gCSBSA

SASA

actionMain

1)(

.

..

.

Re '

qmpd

d

aCk

dt

darSPSPactionPoisoning

'

.Re

PSPtoSP CCCkr )( ..

PSPtdSPSP CCCkr

dt

dC)( .0.

.

)( .0 SPt CC )( pC

Time-on-Stream

Am

ount

of p

oiso

ningactivity

coke

metals

Cat

alyt

ic a

ctiv

ity

I IIIII

Initially high rate of deactivation

• mainly due to coke deposition

Subsequently coke in equilibrium

• metal deposition continues

Typical Stability Profiles in Hydrotreating

Fouling of Catalyst Physical (mechanical) deposition of species from fluid phase onto

the catalyst surface which results in activity loss due to blocking of sites and/or pores

Common to reactions involving hydrocarbons

A carbonaceous (coke) material being deposited on the surface of a catalyst

Coke Deposited can be measured TGA or DTA Monitoring the evolution of CO2 and H2O

Position of Deposited Coke

Preventing of Coking

Optimum catalyst composition

Equilibrium must be in between rate of coke production and rate of coke removal

Coking can be reduced by running at elevated pressure and hydrogen-rich streams.

E.g in catalytic reforming processes

Catalyst deactivated by coking can usually be regenerated by burning off the carbon.

Sintering of Catalyst A loss of active surface area resulting from the prolonged exposure to high

gas-phase temperatures Occurs in both supported and unsupported metal catalyst Two models for crystallite growth due to sintering

Atomic Migration Crystallite Migration

particles migrate coalescesurface

vapour

metastable

migrating

stable

Sintering of Alumina upon Heating

Tcalc (K)

SB

ET (

m2 /

g)

Sintering

Reduction of surface area

Catalyst Deactivation

Separable kinetics

Commercial reactors maintain constant production rate by increasing T (reaction rate constant increases), as catalyst decays (catalyst activity a decreases).

Experimental analysis of the decay rate is as:

catalyst freshrhistory catalystar 'A

"A

0'A

'A trtrta i

'A CfTkr

idd ChTkapdtda

r

Catalyst Deactivation

Sintering (aging) Activity loss by loss of active surface caused by prolonged

exposure to elevated gas-phase reaction temperatures. Mechanistically…

Crystal agglomeration/growth, reducing internal surface area accompanied by narrowing/blocking of pore cross section.

Change in surface structure through recrystallization or other modes of defect elimination (active site loss).

Typically a 2nd order process;

2dd ak

dtda

r t

0d

a

1

2 dtkdaa tk1

1ta

d

Catalyst Deactivation Fouling/Coking

Deposition of carbonaceous material on catalyst surface Catalyst activity level is a function of the amount of carbon

deposited on the catalyst surface (Cc):

where A and n are fouling parameters dependent on the type of gas being processed.

Activity is expressed as f(Cc) by one of the following:

nc AtC

npppc tA1

1C1

1ta

c1Cea

cCa

21

1

Kinetics of Uniform Poisoning

Fundamental to his development, is the assumption that the catalytic site that has adsorbed poison on it is completely inactive. If C,, is the concentration of sites covered with poison the fraction of sites remaining active, called the deactivation or activity function, is represented by

This deactivation function is based on the presumed chemical events occurring on the active sites, and can be related to various chemisorption theories. The overall observed activity changes of a catalyst pellet can also be influenced by diffusional effects, etc., but the deactivation function utilized here will refer only to the deactivation chemistry, to which these other effects can then be added.Since C,, is not normally measured, it must be expressed in terms of the poison concentration, Cp, in the gas phase inside the catalyst. Wheeler used a linear relation